Figure out a genre and basic story line for your "film." That has a big bearing on the kind of poster you will produce. If it's a comedy, you want the poster to be friendly and upbeat. An epic should look grandiose and larger than life. A horror movie should have a dark and spooky poster. Once you've determined the tone of your film, you can pick an image or images that fit it.
Prepare a blank slate in your design software program for your poster. Movie posters average 39 inches high by 27 inches long. If you're printing your poster, you will likely want to make it smaller (you can leave it that height if you wish it to remain a purely electronic image), but maintain that ratio of height to length regardless. Set space aside in the bottom quarter for the title and credits, and a little bit of space at the top for the names or any stars and a tagline. Any picture you choose should fit within the remaining space, or else have sufficient empty space (full of unimportant details) at the very top and the bottom quarter to accommodate the text easily.
Cut and paste the picture you want to use for the poster into the blank slate. The picture can be as simple or as complex as you like--a simple image intended to convey the movie's essence (or stars), or a composite of several images blended to create a new whole.
Add the names of your stars along the top of the frame, spacing them uniformly and taking care to leave an inch or so of space between them and the edge of the frame. Use a font that fits the tone of the movie and make sure the text is bright enough or dark enough to jump off the page from the background.
Insert a tagline at the top of the picture, below the stars' names or at the bottom just above the spot where you intend to put the title. Or instead of a tagline, you can add phony quotes from critics praising your fictitious movie.
Create a title for your movie and place it on the lower end of the poster. The title should be centered within the frame and be large enough to draw the eye. The font should match (or at least be close to) the fonts used in Steps 4 and 5, and like them, it should be in a color that contrasts with the picture behind it so that it stands out all the more. Make sure that the picture space immediately behind it is filled with incidental or unimportant details. There should be enough space beneath it for a cast-and-credits list.
Type in the cast and credits beneath the title, taking care to match the same font used in earlier steps and have the text stand out from the background colors. Add a rating symbol from the MPAA--such as G, PG or R--in the left or right corner.
Revise your poster as you wish, conducting a spell check to make sure you haven't misspelled anything and a visual check to look for any graphic elements that are out of place.
Save your poster to the hard drive, then print out a copy on your color printer.